1. Field of the Invention
The present invention relates generally to inductively coupled electromagnetic energy field transmission and detection systems, such as a transponder in combination with an interrogation system, and more particularly to an impedance matching coil assembly for use in such a transponder.
2. Brief Discussion of the Prior Art
Many objects, such as houses, pets and cars, require some means of identification. Many prior art methods of identification have required visual interrogation of the identifying medium to extract the identification data, such as reading numbers on houses, license plates on cars, and collar tags or brands on animals. Electronic identification tags have also been created, which can be associated with the object and electronically communicated with at a distance, such as the electronic sensing and actuator systems shown in U.S. Pat. Nos. 3,732,465 and 3,752,960.
The systems described in those patents are comprised of an active element, having a single transmitting and receiving coil, which operates by transmitting an electromagnetic field within the proximity of a passive electrical circuit, thereby inductively coupling the coil with the passive circuit. The passive circuit operates to create a characteristic change in the transmitted electromagnetic field that can be detected by the electronics associated with the receiving coil and used to trigger some mode of operation for the system. Although such systems remove some of the restrictions associated with the previously described visual identification systems, such system are really nothing more than electronic keys, and actually convey less information to the active element than does a visually inspectable tag.
More sophisticated electronic systems use an exciter coil to transmit a high intensity electromagnetic energy field in the proximity of an electronic tag or transponder. The transponder is energized by the electrical energy inductively transferred by the transmitted magnetic field and is made operative to output a modulated identification signal which can be detected by an inductively coupled receiving coil proximately located at the exciter coil. The passive tag or transponder element of many of these devices, such as that described in U.S. Pat. No. 4,262,632, includes a coil which receives electromagnetic energy from a transmitted interrogation signal and retransmits an identification signal developed by the identification circuitry of the transponder. Electrical control circuitry within the transponder converts and rectifies the energy received from the coil and develops a dc power source for use in operating the transponder's identification circuitry.
Transponders which utilize a sufficiently large capacitor or resident power source, such as a battery, are able to transmit identification signals over fairly large distances, from a few yards to thousands of feet. However, in certain applications, such as identifying small animals, the size of the sealed packaging required to house the battery or capacitor may be too large for the intended use.
A great amount of effort has been expended to develop a truly passive transponder, which will do more than simply operate as an electronic key. A transponder which can be safely implanted within livestock and interrogated from a practical distance would help to reduce problems associated with certain regulations being implemented by the European Economic Community that will require all livestock to be separately identified.
One effort to create such a transponder resulted in the syringe-implantable transponder of European Patent No. 258,415. This patent and other related patented systems, such as U.S. Pat. Nos. 3,859,624, 3,689,885, 4,532,932 and 4,361,153, disclose passive elements which operate in real time and therefore do not require any type of significant energy storage means. The transmitting and receiving units of these systems, which are often referred to as interrogators or readers, typically include either a single, dual or triple coil arrangement, which is used to both transmit a high intensity electromagnetic field in proximity of the identification unit and receive an identification signal retransmitted by the transponder in proximity of reader coils of the interrogator.
The detection range of such systems is typically very restricted since the strength of the electromagnetic field produced by the transponder drops by l/d.sup.5 at the receiver, where d is the distance between the receiving coil and the transponder coil, as the interrogator is moved away from the transponder. In fact, it has been calculated that at distances of 6 to 8 inches the strength of the magnetic field carrying the identification signal from the transponder, in devices similar to that described in the European Patent No. 258,415, is only an estimated one billionth the strength of the magnetic field carrying the interrogation signal to the transponder. Hence, the interrogator must normally be placed in very close proximity to the transponder in order for the identification signal to be detected. This limitation, of course, greatly restricts the utility of such devices, since not all objects may be so closely approached in order to be read.
Although the specification of European Patent No. 258,415 states that the system disclosed therein is operative to detect the retransmitted signal at distances on the order of inches, it has been found that devices constructed in accordance with that specification are actually incapable of obtaining such a range and are generally only effective when positioned within about an inch of the transponder. An identification device which is limited to operation within such a small range is of limited usefulness, especially when it is to be used to identify large or wild animals or other objects which cannot be readily approached. The problems with this system do not, however, so much rest with the encapsulation configuration or identification circuitry which is described in the patent and hereby incorporated for the purpose of describing the present invention, but rather in the manner in which the interrogator and transponder are coupled so as to transfer energy and information between one another.
Not only do these devices fail to provide for efficient coupling between the interrogator and transponder, the signals which are transmitted by the transponder and received by the interrogator are highly susceptible to interference and noise which severely affects the integrity of the detection portion of the system. This latter problem understandably increases in severity as the transponder is moved away from the interrogator, since it becomes more and more difficult to distinguish between the high intensity transmission field and the low intensity transponder field as the transponder field drops in strength.
Hence, a need has arisen for a transmission and detection system which can simultaneously transmit a high energy magnetic field, sufficient to power the transponder unit, and detect a localized retransmitted magnetic field at greater distances and with greater reliability. A need has also arisen for a transponder coil assembly for use in such systems which maximizes the quantity of energy which can be transferred by the inductive coupling between the identification circuitry and the interrogation and reception fields.